European patent 0,248,758 describes a microwave applicator which is inserted in the rectum for treating the prostate by hyperthermia. The microwave applicator is kept in position inside the rectum by a laterally projecting balloon, meaning that special protection of the rectum is required. Preferably, for heating the prostate, means are provided for reflecting the emitted microwaves which are then concentrated at the side of the body opening opposite to the balloon. Ducts are provided in the microwave emitting rectal applicator for circulating a cooling liquid (see page 5, lines 1 to 5).
The cooling liquid protects the rectal walls in contact with the microwave antenna from damage. Moreover, the presence of the reflecting means is absolutely necessary to prevent the antenna from emitting in all directions, when the radiation is only required to be directed onto the prostate tissues.
European patent 0,246,176 describes a catheter having an antenna equipped with a temperature sensor which is inserted into the urethra through to the bladder, for controlling the temperature reached in the urethra through use of the microwave applicator described in the above-cited European patent 0,248,558.
From a practical point of view, the design of the microwave applicator described in European patent 0,248,758 for introduction in the rectum, is not satisfactory in that the application lacks accuracy since the prostate is remotely-irradiated.
It is furthermore essential with this method to prevent the emission of microwaves in all directions, this implying an emitting antenna of very special design equipped with reflecting means for directing the radiated microwaves in only one direction, namely towards the prostate.
Also, and unquestionably, with such a device, it is quite difficult, if not impossible, to protect the sphincter, and more generally to protect the surrounding tissues. This being especially so in the case of bladder treatment.
Finally, no working frequency is disclosed.
European patent 0,105,677 describes such a microwave antenna device for hyperthermia therapy designed to be introduced in the tract or the lumen, in which device the antenna assembly is placed inside a thin supple polymer film, without any means of protecting the sensitive tissues from the heat. The working frequency is broadly disclosed as ranging between 300 and 3,000 MHz, for instance 915 MHz (page 4, lines 7 and 24).
Patent document WO-A-81/03616 to Bicher describes yet again a microwave antenna system for hyperthermia therapy, which is inserted into a body cavity and used for the treatment of cancer and similar. Typical examples of inserting apertures are the mouth and upper throat.
The introductory part of said patent document cites the document J. Microwave Power 14(2): 167-171 (1979) relative to an applicator working at a frequency of 400 or 915 MHz, introduced through the rectum for irradiation of the prostate. It is specified that said irradiation treatment involves a burning of the tissues which are close to the electrode.
According to said document, the characteristic of this irradiation treatment is that it is carried out at a lower frequency and at a higher wavelength to improve penetration into the tissues to depths of 5 to 6 cm. Such deep penetration permits extensive irradiation of large tumors in the bowels or similar.
The revue "Proceedings of the 13th Annual Northeast Bioengineering Conference, Philadelphia, Mar. 12-13, 1987, pages 390-393, Ryan" describes the pretreating by microwave-induced hyperthermia of patients suffering from biliary obstruction due to cancer, by inserting a catheter into the choledoch to allow drainage in an invasive way. The catheter works at 915 MHz.
Tests have been carried out with the same antenna in various media, as reported in FIG. 5. In the case of tests B and D, the medium was a saline solution, and it is underlined in the "Discussion" part that when the medium is changed from air to a conductive medium such as a saline or bile medium, the performances decrease dramatically, which rather discourages the specialist from using a saline solution in microwave-induced hyperthermia.
Also, it is known from Proceedings of the IEEE, 72 (1984), February, No. 2, 224-225, the advantage of using focused microwave hyperthermia applicators. It is also disclosed obtention of an optimal frequency for a fixed aperture size and a given penetration depth. FIG. 2 discloses optimal frequencies for given penetration depth of 2, 4 and 6 cm at 3,000; 1,500 and 915 MHz.